Lithographic processing cell and device manufacturing method

ABSTRACT

A double processing technique for device manufacture is disclosed that includes performing a first patterning step to form apertures in a resist layer which apertures are filled before the first resist layer is stripped and replaced by a second resist layer to be used in the second exposure. In an embodiment, a lithographic cell includes an integrated fill and/or strip unit and may be used to perform the above technique.

FIELD

The present invention relates to a lithographic processing cell,including a lithographic apparatus and a process apparatus, and to oneor more device manufacturing methods.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”—direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a device manufacturing method using projection lithographicapparatus, the minimum feature size, often referred to as criticaldimension (CD), is determined by the wavelength (λ) of the exposureradiation and the numeric aperture (NA) of the projection system.Various techniques have been developed to reduce CD and these arecommonly combined into a factor known as k₁ so that CD=k₁. λ/NA. Withcurrent technology, it may not be possible to image a feature smallerthan determined by a k₁ factor of about 0.25 in a singleexposure-develop-process cycle. However a smaller feature may be imagedwith a double exposure and double process technique.

In a double exposure technique, a single layer of resist is exposedtwice—either with two different patterns or with the same pattern butwith a positional offset—before being developed. In a double processtechnique, a first layer of resist is exposed and developed and then thesubstrate is etched, transferring the pattern to the substrate, then asecond layer of resist is applied to the substrate. The second layer isthen exposed, developed and the substrate etched so that the finalpattern in the substrate is produced by the combination of the two etchsteps. A double process technique may be used to create a wide varietyof useful structures but may be slow, taking one or two days because ofthe etch steps and the need to remove the substrate from thelithocell—comprising a lithographic apparatus and process apparatus suchas a spin coater, developer and bake & chill plates—to perform the etchsteps. Although a double exposure technique may be performed much morequickly, the range of structures it may be used for is more limited.

SUMMARY

It is therefore desirable, for example, to provide an improved methodand apparatus capable of creating structures equivalent to a k₁ value ofless than or equal to 0.25.

According to an aspect of the invention, there is provided alithographic cell, comprising:

a lithographic apparatus;

a plurality of process apparatus, the plurality of process apparatuscomprising a fill apparatus, or a strip apparatus, or both a fillapparatus and a strip apparatus; and

a control unit configured to control both the lithographic apparatus andthe process apparatus.

According to an aspect of the invention, a device manufacturing methodusing a lithographic apparatus, the method comprising:

filling first apertures in a first pattern in a first layer ofradiation-sensitive material of a substrate with a first filler;

removing the first layer of radiation-sensitive material withoutremoving the first filler;

applying a second layer of radiation-sensitive material around the firstfiller;

exposing and developing the second layer of radiation-sensitive materialto form second apertures in a second pattern therein;

filling the second apertures with a second filler; and

removing the second layer of radiation-sensitive material withoutremoving the first or second fillers.

According to an aspect of the invention, a device manufacturing method,the method comprising, in a lithographic cell comprising a lithographicapparatus and a plurality of process apparatus:

filling first apertures in a first pattern in a first layer ofradiation-sensitive material of a substrate with a first filler;

removing the first layer of radiation-sensitive material withoutremoving the first filler;

applying a second layer of radiation-sensitive material around the firstfiller; and

exposing the second layer of radiation-sensitive material to an imagecorresponding to a second pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell including the apparatus of FIG. 1;

FIGS. 3 to 12 depict stages in a manufacturing method according to anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment markers M1,M2 and substrate alignment markers P1, P2. Although the substratealignment markers as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment markers). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment markers may be located between the dies. A beamcharacteristic or other measurement may be using a transmission imagesensor TIS provided on the substrate table WT.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

The lithographic apparatus LA forms part of a lithographic cell LC, alsosometimes referred to a lithocell or lithocluster, which also includesapparatus to perform pre- and post-exposure processes on a substrate.Conventionally these include one or more spin coaters SC to deposit aresist layer, one or more developers DE to develop exposed resist, oneor more chill plates CH and one or more bake plates BK. A substratehandler, or robot, RO picks up a substrate from input/output ports I/O1,I/O2, moves it between the different process apparatus and then deliversit to the loading bay LB of the lithographic apparatus. These devices,which are often collectively referred to as the track, are under thecontrol of a track control unit TCU which is itself controlled by asupervisory control system SCS, which also controls the lithographicapparatus. The control unit may control, for example, the illuminatorand the projection system to expose a substrate. Thus, the differentapparatus may be operated to maximize throughput and processingefficiency.

According to an embodiment, the track is supplemented by a fillapparatus FI and a strip apparatus ST. The fill apparatus FI is arrangedto coat a substrate with a layer of filler material that fills one ormore apertures in the exposed and developed resist. The fill apparatusmay be a spin coater of known type, for example, provided with a supplyof a filler material, which may be selected from the group consisting ofspin-on SiO2, spin-on silicon nitride, spin-on silicon oxynitride, andsilicon polymer. A spin coater used to apply resist and provided asstandard in a track may be used as the fill apparatus if connected to asupply of suitable material. A multi-purpose coater, connected tosupplies of several different materials so as to apply a selected one ofthe materials to a substrate may also be used. Desirable properties ofthe filler material are that it should be easy to apply in layers ofcontrollable thickness, should flow when applied to fill apertures in anexposed and developed resist layer and should be resistant to a methodto remove the exposed and developed resist. For example, the filler maybe substantially insoluble in a solvent used to dissolve the developedresist, or at least have a substantially lower solubility than thedeveloped resist.

The strip apparatus ST may be a washing device connected to a supply ofsolvent or reagent that dissolves or reacts with the filler material soas to remove it without substantially affecting the underlying layer ofthe substrate. The strip apparatus may be a standard developer deviceadapted by connection to a supply of a suitable reagent or solvent.Again, a multi-purpose device may be used.

According to an embodiment of the invention, the above describedlithographic cell is used to perform a double processing technique, asdescribed below. It should be noted that the method described below maybe performed on other apparatus, e.g. in which one or both of the stripand fill apparatus are not integrated into the track, but use of theabove described apparatus may enable the method to be described to beperformed with higher throughput and higher yield. Use of the abovedescribed apparatus may avoid the need for a substrate to be removedfrom the lithographic cell between exposures, hence reducing cycle timeand ensuring greater uniformity between exposures. In particularsubstrate handling steps may be avoided.

A double processing method according to an embodiment of the inventionis described with reference to FIGS. 3 to 12, which are cross-sectionsthrough a substrate to which the method of the embodiment of theinvention is applied. First a substrate 10, e.g. a silicon wafer, iscoated with a first layer of radiation-sensitive material 11 (e.g.resist), as shown in FIG. 3. This is exposed and developed so as to formapertures 11 a at a pitch P1, which is, in an embodiment, close to theminimum pitch imagable by the lithographic apparatus used, as shown inFIG. 4. If the resist is a positive tone resist, this step may beperformed using a patterning device (e.g., mask) having bright features,e.g. lines or studs, on a dark background and developing away theilluminated parts of the resist.

In a conventional double patterning technique, the substrate is thenetched so that the resist pattern is transferred into the substrate.However, in the present embodiment, the apertures in the resist patternare instead filled with a filler material 12 of the type described aboveas shown in FIG. 5. As shown in FIG. 6, the remaining parts of the firstresist are removed, so that the first filler 12 forms islands or mesas,and a second layer of resist 13 is coated around them, as shown in FIG.7. The first layer of resist cannot in general be re-used for a secondexposure because it will retain the memory of having been exposed, itwill be sub-threshold in places other than where the apertures 11 a wereformed and a second exposure would require too high a contrast toprovide a useful process window.

Following a second exposure and development step, the situation shown inFIG. 8 is reached: a second set of apertures 13 a at pitch P1 are formedin the second resist layer interleaved with the first filler islands 12.The second exposure step may be performed with the same pattern as usedfor the first exposure, but with a positional offset, or may use adifferent pattern. The pitch P1 in FIG. 4 need not be same value aspitch P1 in FIG. 8. Further, although the Figures are drawn that way, itis not essential that the second apertures 13 a are the same size as thefirst filler islands 12 nor need they be spaced equidistantly betweenfirst filler islands 12.

Next the second apertures 13 a are filled with a second filler material14 to reach the situation shown in FIG. 9. The second filler materialmay be the same as first filler material. Then, the second resist 13 isremoved so that, as shown in FIG. 10, the first and second fillermaterials 12, 14 form islands spaced apart on top of the substrate 10.The final steps are then to transfer the pattern defined by the spacesbetween the first and second filler islands 12, 14 into the substrate10, FIG. 11, and to remove the filler materials, FIG. 12. The patterntransfer step may be, for example, an etch to form recesses 15 in thesubstrate 10, or any other process that alters or adds to the substrateonly in the spaces defined between the filler islands 12, 14. It will beseen, for example, that the features 15 formed by the pattern transferstep have a pitch P2 that is half the pitch P1 of the features definedin the imaging steps. Thus, features may be patterned with an effectivek₁ value of less than 0.25 using two imaging steps with a k₁ value ofgreater than 0.25.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic cell, comprising: a lithographic apparatus; aplurality of process apparatus, the plurality of process apparatuscomprising a fill apparatus, or a strip apparatus, or both a fillapparatus and a strip apparatus; and a control unit configured tocontrol both the lithographic apparatus and the process apparatus. 2.The lithographic cell of claim 1, wherein the fill apparatus is a spincoater.
 3. The lithographic cell of claim 1, wherein the strip apparatusis a developer connected to a supply of solvent or reagent able toselectively remove a filler material.
 4. The lithographic cell of claim1, wherein the control unit comprises a storage medium having storedtherein instructions to cause the lithographic cell to perform a methodcomprising: filling first apertures in a first pattern in a first layerof radiation-sensitive material of a substrate with a first filler;removing the first layer of radiation-sensitive material withoutremoving the first filler; applying a second layer ofradiation-sensitive material around the first filler; and exposing thesecond layer of radiation-sensitive material to an image correspondingto a second pattern.
 5. The lithographic cell of claim 4, wherein thestorage medium further comprises instructions to cause the lithographiccell to perform a method comprising: developing the second layer ofradiation-sensitive material to form second apertures in the secondpattern therein; filling the second apertures with a second filler; andremoving the second layer of radiation-sensitive material withoutremoving the first or second fillers.
 6. The lithographic cell of claim5, wherein the storage medium further comprises instructions to causethe lithographic cell to perform a method comprising: transferring athird pattern, defined by the areas of the substrate not covered by thefirst and second fillers, into the substrate; and removing the first andsecond fillers.
 7. The lithographic cell of claim 4, wherein the storagemedium further comprises instructions to cause the lithographic cell toperform a method comprising: applying the first layer ofradiation-sensitive material to the substrate; and exposing anddeveloping the first layer of radiation-sensitive material to form thefirst apertures in the first pattern therein.
 8. The lithographic cellof claim 1, wherein the lithographic apparatus comprises: an illuminatorconfigured to condition a beam of radiation; a support structureconfigured to hold a patterning device, the patterning device configuredto pattern the radiation beam; a substrate table configured to hold asubstrate; and a projection system configured to project the patternedradiation beam onto a target portion of the substrate.
 9. Thelithographic cell of claim 8, wherein the control unit is configured tocontrol the illuminator and the projection system to expose thesubstrate.
 10. The lithographic cell of claim 1, comprising a spincoater as the fill apparatus and a developer as the strip apparatus. 11.A device manufacturing method using a lithographic apparatus, the methodcomprising: filling first apertures in a first pattern in a first layerof radiation-sensitive material of a substrate with a first filler;removing the first layer of radiation-sensitive material withoutremoving the first filler; applying a second layer ofradiation-sensitive material around the first filler; exposing anddeveloping the second layer of radiation-sensitive material to formsecond apertures in a second pattern therein; filling the secondapertures with a second filler; and removing the second layer ofradiation-sensitive material without removing the first or secondfillers.
 12. The method of claim 11, further comprising: transferring athird pattern, defined by the areas of the substrate not covered by thefirst and second fillers, into the substrate; and removing the first andsecond fillers.
 13. The method of claim 12, wherein the first patternand the second pattern are interleaved so that the pitch of features inthe third pattern is smaller than the pitches of features in the firstand second patterns.
 14. The method of claim 13, wherein transferringthe third pattern into the substrate comprises etching the areas of thesubstrate not covered by the first and second fillers.
 15. The method ofclaim 11, further comprising: applying a first layer ofradiation-sensitive material to a substrate; and exposing and developingthe first layer of radiation-sensitive material to form the firstapertures in the first pattern therein.
 16. The method of claim 11,wherein filling the first and second apertures comprises spin coating afiller material onto the substrate over the exposed and developed layerof radiation-sensitive material.
 17. The method of claim 11, wherein thefirst filler and the second filler are formed of the same material. 18.A device manufacturing method, the method comprising, in a lithographiccell comprising a lithographic apparatus and a plurality of processapparatus: filling first apertures in a first pattern in a first layerof radiation-sensitive material of a substrate with a first filler;removing the first layer of radiation-sensitive material withoutremoving the first filler; applying a second layer ofradiation-sensitive material around the first filler; and exposing thesecond layer of radiation-sensitive material to an image correspondingto a second pattern.
 19. The method of claim 18, further comprising:developing the second layer of radiation-sensitive material to formsecond apertures in the second pattern therein; filling the secondapertures with a second filler; and removing the second layer ofradiation-sensitive material without removing the first or secondfillers.
 20. The method of claim 19, further comprising: transferring athird pattern, defined by the areas of the substrate not covered by thefirst and second fillers, into the substrate; and removing the first andsecond fillers.
 21. The method of claim 18, further comprising: applyingthe first layer of radiation-sensitive material to the substrate; andexposing and developing the first layer of radiation-sensitive materialto form the first apertures in the first pattern therein.